1. Field of the Invention
This invention relates to a method for ozonation of water with ozone generated in oxygen feed gas by silent electric discharge, and contacting the water and ozone-oxygen gas.
2. Description of the Prior Art
The water disinfection capabilities of ozone have long been recognized. Ozone has been used as a germicide for the sterilization of drinking water, and also for the tertiary treatment of a treated wastewater stream or effluent from an activated sludge or trickling filter plant. However, the high cost of ozone relative to the more economical alternative, chlorination, has generally limited its use. Nonetheless, the use of ozone in water treatment for example, has been increasing in recent years due to several of its unique properties which offer substantial advantages. For example, ozone is a very powerful oxidizing agent and many ozone reactions are much more rapid than chlorine reactions. Also, ozone has been shown to be a highly efficient germicide and virucide, and fortuitously leaves a beneficial oxygen residual as one reaction product. The advantages stemming from a residual dissolved oxygen content (D.O.) are well-known. Finally, the oxidized products remaining after ozonation are generally less toxic than the corresponding chlorinated or unoxidized products remaining after treatment with chlorine.
Most commercial ozone generators are of the silent electric discharge type. Such generators exhibit sharp variations in efficiency depending on the oxygen concentration of the feed gas and the ozone concentration of the generator product gas. For instance, although the production of ozone-in-oxygen gas containing about 4% ozone by weight is possible with many commerical ozone generators when operating with a high purity oxygen feed gas, i.e., about 90% oxygen (by volume), the generation of an ozone-in-oxygen-containing product gas with ozone concentrations above about 2% by weight is normally accompanied by a substantial economic penalty.
One commercially available ozone generator having ozone generation power characteristics generally superior to most commercial ozone generators is described and claimed in the following U.S. Pat. Nos. 3,784,898; 3,798,457; 3,875,035; 3,891,561; 3,899,682; 3,903,426; 3,954,586; 3,984,697; and 3,996,474 all to F. E. Lowther. This generator will be hereinafter referred to as the Lowther generator. However, even with this improved generator, operation in excess of 2-4% ozone concentration (by weight) in the oxygen-enriched product gas requires a substantial additional power expenditure and has not generally been practiced.
In addition to the cost of producing the ozone-in-oxygen-containing gas, an overall ozone contacting system must also include an air separation system for producing the oxygen-enriched feed gas for the ozone generator. Since most commercial ozone generators are typically operated to produce an ozone-in-oxygen-containing gas having about 1-4% ozone by weight, only a small fraction (i.e., about 4%) of the oxygen in the oxygen-enriched feed gas for the ozone generator is actually utilized to produce ozone. For this reason, the prior art has proposed a variety of water ozonation systems maximizing the utilization of the oxygen-enriched feed gas.
One such system, described in U.S. Pat. No. 3,660,277 to McWhirter et al, involves an oxygen-aerated secondary wastewater treatment. In this instance the patentees believe that it is advantageous to produce an oxygen-enriched feed gas stream of a sufficient size to satisfy not only the ozonation requirements of a treated wastewater stream, but also to satisfy the oxygen requirements for the secondary treatment of the raw wastewater stream as well. In the McWhirter et al system, the oxygen-enriched feed gas produced by the air separator may be initially divided into two streams. A first gas fraction may be fed directly to the secondary wastewater treatment portion of the system; while the other major, second gas fraction is fed directly into an ozone generator. The ozone-in-oxygen-containing gas produced by the ozone generator having an ozone concentration between 0.5% and 4.0% (by volume), i.e., below about 6% ozone (by weight) is used to disinfect in the ozone contacting stage, the treated wastewater stream (secondary effluent) discharged from the secondary treatment part of the process.
The ozone-depleted, oxygen-enriched gas discharged from the ozone contacting stage is then passed to the secondary treatment part of the system and combined with the optional first gas fraction of the oxygen-enriched feed gas as the oxygen-containing feed gas for secondary wastewater treatment. By using such a system, one can achieve a high overall utilization of the original oxygen-enriched feed gas produced by the air separator, e.g. on the order of 90-95% is possible.
One alternative to the simple integrated wastewater treatment-ozonation system of McWhirter et al is described in U.S. Pat. No. 4,132,637 to Key et al. As in the McWhirter et al system, the air separator in the Key et al system produces sufficient oxygen-enriched feed gas to satisfy both the ozonation requirements of the treated wastewater stream (secondary effluent), as well as the biological oxygenation requirements for the secondary treatment of the raw influent wastewater. However, a portion of the ozone-depleted, oxygen-enriched vent gas from the water contacting is dried and mixed with the portion of the oxygen-enriched feed gas fed directly to the ozone generator.
However, in many instances where ozonation of water is to be employed there will not be an adequate oxygen-aerated wastewater treatment or any other oxygen consumer available. To conserve oxygen-enriched gas in such cases, it has been common practice to recirculate or recycle the ozone-depleted, oxygen-enriched gas discharged from the ozone contacting stage after appropriate treatment steps (e.g. drying) back to the ozone generator for mixing with an appropriate make-up supply of an oxygen-enriched feed gas. This approach will hereinafter be referred to as the "classical recycle method". As will be hereinafter discussed in greater detail, the classical recycle method requires expensive equipment, additional power, and provides typically only 50-60% overall utilization of the oxygen in the feed gas.
It is an object of the present invention to provide a water ozonation method which does not require a secondary use for the ozone-depleted, oxygen-enriched gas from the ozone-water contacting.
Another object of the invention is to provide an improved water ozonation method with no secondary oxygen consumption which, in terms of overall equipment and power requirements plus oxygen utilization, is more effective than the classical recycle method.
A final object of this invention is to provide an ozone generation and water contacting method which is exceedingly simple in design, low in cost and high in reliability.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.